1. Field of the Invention
The present invention relates to a wireless local area network (LAN) network, a mobile station and a base station, and a method of controlling handoff in the wireless LAN network.
2. Description of the Related Art
With the constant development of hardware technology, terminals have become small in size, and to achieve high performance. In combination with this achievement, wireless packet data networks enable users to obtain useful information regardless of time or space. This computing paradigm is based on core technology which allows a terminal to receive information regardless of its current location, that is, technology which allows portability of a terminal and wide-ranging mobility of a user. Thus, mobility-supporting technology generally designates a method used for tracking movement of a terminal between different hardware characteristic areas or different mobile communication networks, and mutually transmitting location information between network components if necessary.
Users of mobile stations must be provided with reliable and stable mobility-supporting technology so that they can enjoy a constant and useful computing environment while watching a multimedia presentation, surfing the Internet, sending email, and so on. In particular, in a wireless LAN environment transmitting data in high-speed, an improved mobility-supporting system together with a dynamic load balancing technique can maintain network connections while remote users pass through different access points.
FIG. 1 illustrates a handoff process in a general wireless LAN network. Referring to FIG. 1, the general wireless LAN network may comprise a plurality of access points 1, 2 and 3 and a moving mobile host 4 that performs handoff process. The access points 1, 2 and 3 periodically broadcast beacon messages. The broadcasted beacon message includes information on the corresponding access points 1, 2 and 3, such as a time stamp, a capability, an extended service set (ESS) identification (ID) and a traffic indication map (TIM).
The mobile host 4 uses the information included in the beacon message to distinguish the different access points 1, 2 and 3 from each other. When a received signal strength (RSS) weakens, the moving mobile host 4 keeps a beacon message having a higher RSS as a beacon message of a current access point among the adjacent access points 1, 2 and 3.
In an active RSS scanning process, the mobile host 4 transmits a probe request to all the adjacent access points 1, 2 and 3. In response to the probe request, the each of access points 1, 2 and 3 transmits a probe response including periodically broadcasted beacon information.
The mobile host 4 selects the access point 3, for example, transmitting the probe response having the highest RSS to determine the access point 3 as a new access point, and transmits a reassociation request to the new access point 3. The message for the reassociation request includes information on the mobile host 4. The new access point 3 transmits a reassociation response including a supporting bit rate, a terminal ID and information required for restarting communication to the mobile host 4. Here, the previous access point 1 is notified of only the reassociation event, is not notified of a current location of the mobile station 4.
According to the above-described handoff process, since the mobile host 4 has closed (or terminated) a connection with the previous access point 1, packets are lost until a link with the access point 3 is established after movement. A time period, during which the mobile host 4 closes the connection to the previous access point 1, and establishes a link with the new access point 3, is referred to as an open period. In the open period, data transmission cannot be performed, thus resulting in data loss.